1. Technical Field
The present invention relates to disk-substrate molding dies and disk-substrate fabricating methods for forming a disk substrate.
2. Description of Related Art
Disk-shaped substrates such as optical-disk substrates and magnetic optical disks are fabricated by charging molten thermoplastic resin into a cavity provided within a die in view of productivity. Fine convex and concave pits and grooves are formed on a stamper made of nickel or the like which is mounted within the die. The stamper comes into contact with the molten thermoplastic resin so that the fine convex and concave pits and grooves on the stamper are transferred to the thermoplastic resin, and thereafter, the molten thermoplastic resin is solidified to provide a desired optical-disk substrate (refer to JP-A No. 8-66945, for example).
In a conventional common die construction disclosed in JP-A No. 8-66945, a fixed-side mirror-surface plate and a movable-side mirror-surface plate, which are controlled in the temperature, are provided at the upper and lower portions of the cavity. A stamper is mounted on the movable-side mirror-surface plate, and the stamper is secured by a stamper holder at its inner side and also secured by an outer ring at its outer side.
On the other hand, as the density is increased, the intervals between fine convex and concave pits and grooves on the stamper are decreased, thus making it more difficult to introduce thermoplastic resin into the fine convex and concave pits and grooves. In order to transfer convex and concave pits and grooves to an optical-disk substrate, it is necessary to raise the temperature of the stamper to above the thermal deformation temperature of the thermoplastic resin. Since it is necessary to further raise the maximum stamper temperature in order to increase the flowability of the thermoplastic resin, it is necessary to further raise the temperature of a heat medium for adjusting the die temperature.
However, if the temperature of the heat medium for adjusting the die temperature is raised to a high temperature, then it will take a long time to cool down the thermoplastic resin charged within the die to a temperature which enables extraction thereof, thus resulting in an increase of the molding time for an optical-disk substrate. Therefore, there has been suggested a die construction employing a heat insulation sheet provided on the back surface of the stamper, in order to make the stamper temperature high even when the heat-medium temperature is low (refer to JP-A No. 62-5824, for example).
Further, there has been disclosed a die construction including low heat-conductivity members provided within an upper die and a lower die, in order to cause the temperature within the upper and lower dies to change in a upper-lower symmetrical manner along the thickwise direction (refer to JP-A No. 7-100866, JP-A No. 9-207141 and JP-A No. 2000-331385, for example).
As the low heat-conductivity members, heat resistant plastics such as polyimide and ceramics have been mainly employed. JP-A No. 62-5824 discloses employing aluminum and cupper as metal low heat-conductivity materials, and JP-A No. 2000-331385 discloses employing bismuth as a low heat-conductivity material.
Further, conventional die constructions have malfunctions as follows.
In a die construction disclosed in JP-A No. 7-100866, a fixed-side stamper and a movable-side stamper include a slow-cooling plate. The fixed-side stamper and the movable-side stamper are secured by an inner holder at their inner sides and are secured by outer rings at their outer sides. Namely, the outer surfaces of the slow-cooling plates are protected by the outer rings. The movable-side stamper is configured and sized to be greater than the fixed-side stamper, and therefore the movable-side outer ring is positioned outside the fixed-side outer ring. Consequently, the fixed-side outer ring holds the fixed-side stamper and also comes into contact with the outer peripheral portion of the movable-side stamper. Namely, the fixed-side outer ring is sandwiched between the fixed-side stamper and the movable-side stamper. Further, the fixed-side outer ring defines the outer peripheral side surface of an optical-disk substrate. As a result, if the resin charge pressure overcomes the die fastening pressure, then the die is opened from the contacting surface of the movable-side stamper. Consequently, resin enters into the gap between the fixed-side outer ring and the movable-side stamper, thus forming burrs on the outer peripheral side surface of the optical disk substrate.
In a die construction disclosed in JP-A No. 9-207141, a nest mounted on a nest-mounting portion of the die is secured by an inner pressing plate at its inner peripheral portion and is secured by an outer pressing plate at its outer peripheral portion. However, since a step is formed at the outer peripheral portion of the nest by the outer pressing plate, there is caused the problem that fine convex and concave pits and grooves can not be formed at the outer peripheral portion of the formed optical disk substrate.
A die construction disclosed in JP-A No. 2000-331385 is configured such that a movable die is slidably fit in a concave-shaped portion of a fixed die. A heat insulation member is mounted on each of the movable die and the fixed die. When the movable die slides, the outer peripheral surface of the heat insulation member mounted on the movable die is slid while being fit against the inner peripheral surface of the concave-shaped portion of the fixed die, and therefore the heat insulation member is prone to be exfoliated from the outer peripheral side surface.
In a die construction disclosed in JP-A No. 9-262838, a heat insulation layer made of a heat insulation polymer and a metal layer are extended from the cavity surface of the base die to the side surface. However, the heat insulation layer and the metal layer provided on the side surface come into contact with a fixed-side mounting plate which is faced thereto and also constitute split surfaces which can be split into two surfaces. Consequently, the heat insulation layer and the metal layer provided on the side surface of the base die will not serve as a sliding portion during the molding operation.
Further, although plastic materials, ceramic materials and metal materials are conventionally employed as low-heat conductivity materials, these low heat-conductivity materials have problems as follows.
Namely, plastic materials generally have poor stiffness and poor surface strengths. Ceramic materials are generally brittle and thus have poor impact resistances. Aluminum and cupper (metal materials) disclosed in JP-A No. 62-5824 have heat conductivities higher than that of the stainless steel constituting the die and therefore can not serve as low heat-conductivity materials. Bismuth disclosed in JP-A No. 2000-331385 is brittle and has poor hardness and therefore has undesirable mechanical characteristics.